The invention relates to a steam generator with a combustion chamber for fossil fuel. A horizontal gas flue and a vertical gas flue are provided downstream of the combustion chamber.
In a power plant with a steam generator, the energy content of a fuel is utilized for evaporating a flow medium in the steam generator. For the evaporation of a flow medium, the steam generator has evaporator tubes, the heating of which leads to the evaporation of the flow medium carried in them. The steam supplied by the steam generator may, in turn, be provided, for example, for a connected external process or else for driving a steam turbine. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. In the case of a generator, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
In this context, the steam generator may be configured as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper xe2x80x9cVerdampferkonzepte fxc3xcr Benson-Dampferzeugerxe2x80x9d [Evaporator concepts for Benson steam generators] by J. Franke, W. Kxc3x6hler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to the evaporation of the flow medium in the steam generator tubes in a single pass.
Steam generators are conventionally configured with a combustion chamber in a vertical form of construction. This means that the combustion chamber is configured for the heating medium or fuel gas to flow through in an approximately vertical direction.
At the same time, the combustion chamber may be followed on the fuel-gas side by a horizontal gas flue, the fuel-gas stream being deflected into an approximately horizontal flow direction at the transition from the combustion chamber into the horizontal gas flue. However, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type generally require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the production and the assembly of the steam generator, this outlay being the greater, the greater is the overall height of the steam generator.
The configuration of the containment wall of the gas flue or combustion chamber of the steam generator presents a particular problem with regard to the tube-wall or material temperatures which occur there. In the subcritical pressure range up to about 200 bar (20 MPa), the temperature of the containment wall of the combustion chamber is determined essentially by the level of the saturation temperature of the water when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, by the use of evaporator tubes which have a surface structure on their inside. In particular, internally ribbed evaporator tubes come under consideration in this respect, of which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
Experience has shown that it is not possible to avoid containment walls of the combustion chamber being heated to a differing extent. As a result of the different heating of the evaporator tubes, therefore, the outlet temperatures of the flow medium from evaporator tubes heated to a greater extent are substantially higher than in the case of evaporator tubes heated normally or heated to a lesser extent. This may give rise to temperature differences between adjacent evaporator tubes, leading to thermal stresses which may reduce the useful life of the steam generator or even cause pipe cracks.
It is accordingly an object of the invention to provide a steam generator for fossil fuel which overcomes the abovementioned disadvantages of the heretofore-known steam generators of this general type and which requires a particularly low outlay with respect to its production and its assembly and in which, at the same time, temperature differences between adjacent evaporator tubes, when the steam generator is in operation, are kept particularly low.
With the foregoing and other objects in view there is provided, in accordance with the invention, a steam generator, including:
a combustion chamber for fossil fuel, the combustion chamber having a fuel-gas side;
a horizontal gas flue;
a vertical gas flue connected, via the horizontal gas flue, on the fuel-gas side to the combustion chamber;
the combustion chamber having burners provided substantially on a level with the horizontal gas flue;
the combustion chamber having containment walls formed from vertically disposed evaporator tubes welded to one another in
a gastight manner;
a number of the evaporator tubes being subdivided into a first group and a second group, the first group and the second group of the evaporator tubes to be acted upon in each case in parallel by a flow medium;
the second group being provided in series with the first group and downstream of the first group as seen in a direction of flow of the flow medium;
the containment walls of the combustion chamber being subdivided into a first region and a second region along a main flow direction of a fuel gas flow;
the first region being formed from evaporator tubes of the first group and the second region being formed from evaporator tubes of the second group; and
the second region being provided, on the fuel-gas side, between the first region and the horizontal gas flue.
In other words, the object of the invention is achieved by a steam generator with a combustion chamber for fossil fuel, which is followed on the fuel-gas side, via a horizontal gas flue, by a vertical gas flue, the combustion chamber including a number of burners provided on a level with the horizontal gas flue, wherein the containment walls of the combustion chamber are formed from vertically disposed evaporator tubes welded to one another in a gastight manner, a number of the evaporator tubes are subdivided into a first group and a second group, the first group and the second group of the evaporator tubes can be acted upon in each case in parallel by a flow medium, and the second group follows the first group of the evaporator tubes in series in the direction of flow of the flow medium, and in which the containment walls of the combustion chamber are subdivided into a first region and a second region in the main direction of flow of the fuel gas, the first region is formed from evaporator tubes of the first group and the second region from evaporator tubes of the second group, and the second region is provided, on the fuel-gas side, between the first region and the horizontal gas flue.
The invention is based on the idea that a steam generator which is to be set up at a particularly low outlay in terms of production and assembly should have a suspension structure which can be implemented in a simple manner. A framework to be set up at a comparatively low technical outlay for the suspension of the combustion chamber may, in this case, be accompanied by a particularly low overall height of the steam generator. A particularly low overall height of the steam generator can be achieved by the combustion chamber being configured in a horizontal form of construction. For this purpose, the burners are provided on a level with the horizontal gas flue in the combustion chamber wall. The fuel gas therefore flows through the combustion chamber in an approximately horizontal direction when the steam generator is in operation.
Moreover, when the horizontal combustion chamber is in operation, temperature differences between adjacent evaporator tubes should be particularly low in order reliably to avoid premature material fatigue. Where a horizontal combustion chamber is concerned, however, when the continuous-flow steam generator is in operation, the rear region of the combustion chamber, as seen on the fuel-gas side, is heated to a comparatively lesser extent than the front region of the combustion chamber, as seen on the fuel-gas side. Furthermore, for example, an evaporator tube in proximity to the burners is heated to a greater extent than an evaporator tube provided in a corner of the combustion chamber. At the same time, in an extreme case, the heat flow density may be about three times greater in the front region of the combustion chamber than in the rear region.
As regards the hitherto conventional mass flow densities, given in kg/m2s and with respect to a 100% steam power output (full load), of 2000 kg/m2s, the mass throughput decreases in a tube heated to a greater extent and increases in a tube heated to a lesser extent, in each case in relation to the average value of the mass throughput of all the tubes. This behavior is caused by the relatively high fraction of frictional pressure loss in the total pressure drop of the evaporator tubes. Moreover, because of the particularly low height of the horizontal combustion chamber, the relative differences in length of the evaporator tubes are substantially greater than in the case of a vertical combustion chamber. This additionally increases the differences in the heating and in the frictional pressure loss of the individual evaporator tubes. In order nevertheless to ensure approximately identical temperatures between adjacent evaporator tubes, a number of the evaporator tubes of the combustion chamber are advantageously subdivided into a first group and a second group. At the same time, the first group of evaporator tubes connected in parallel on the flow-medium side is connected in series, with respect to the flow medium, with the second group of evaporator tubes connected in parallel on the flow-medium side.
In the case of a series connection of the first group of evaporator tubes with the second group of evaporator tubes, it proves advantageous if the combustion chamber is subdivided into a first and a second region in the main direction of flow of the fuel gas, the first region being formed from evaporator tubes of the first group and the second region from evaporator tubes of the second group. At the same time, the second region is provided, on the fuel-gas side, between the first region and the horizontal gas flue. This is because, when the steam generator is in operation, that inlet portion of the evaporator tubes of the first region which is acted upon by flow medium has a comparatively lower temperature than the inlet portion of the evaporator tubes of the second region. To be precise, due to the series connection of the evaporator tubes, the second region is acted upon by flow medium which has already passed through the first region. The inlet portion of the horizontal gas flue likewise has a comparatively lower temperature than the inlet portion of the second region of the combustion chamber. Due to the combustion chamber being subdivided into regions, the evaporator tubes of which are connected in parallel in the main direction of flow of the fuel gas, temperature differences between adjacent evaporator tubes when the steam generator is in operation are particularly low.
Advantageously, the first group and the second group of evaporator tubes are in each case preceded by a common inlet header system and followed by a common outlet header system for the flow medium. A steam generator produced in this configuration allows reliable pressure compensation between the parallel-connected evaporator tubes and therefore a particularly favorable distribution of the flow medium during the flow through the evaporator tubes.
A containment wall for the combustion chamber is advantageously the end wall, the evaporator tubes can be acted upon in parallel by a flow medium.
The evaporator tubes of the end wall of the combustion chamber advantageously precede the first group of the evaporator tubes of the combustion chamber on the flow-medium side. This ensures particularly favorable cooling of the end wall.
In a further advantageous embodiment, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes can thereby be adapted in the combustion chamber to a heating profile predeterminable on the gas side. The influence thus exerted on the flow through the evaporator tubes keeps temperature differences at the outlet of the evaporator tubes of the combustion chamber low in a particularly reliable way.
For a particularly good transmission of the heat of the combustion chamber to the flow medium carried in the evaporator tubes, a number of the evaporator tubes advantageously have on their inside in each case ribs which form a multistart thread. In this case, advantageously, a pitch angle a between a plane perpendicular to the tube axis and the flanks of the ribs provided on the tube inside is smaller than 60xc2x0, preferably smaller than 55xc2x0.
To be precise, in a heated evaporator tube configured as an evaporator tube without internal ribbing, a so-called smooth tube, the wetting of the tube wall necessary for a particularly good heat transmission can no longer be maintained beyond a specific steam content. In the absence of wetting, there may be a tube wall which is dry in places. The transition to a dry tube wall of this type leads to a kind of heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube-wall temperatures rise particularly sharply at this point. In an internally ribbed tube, however, in comparison with a smooth tube, this heat transmission crisis arises only at a mass steam content  greater than 0.9, that is to say shortly before the end of evaporation. This is attributable to the swirl which the flow experiences due to the spiral ribs. On account of the different centrifugal force, the water fraction is separated from the steam fraction and is pressed onto the tube wall. The wetting of the tube wall is thereby maintained to high steam contents, so that there are already high flow velocities at the location where the heat transmission crisis occurs. This gives rise, in spite of the heat transmission crisis, to good heat transmission and, as a result, to low tube-wall temperatures.
A number of the evaporator tubes of the combustion chamber advantageously have devices for reducing the throughflow of the flow medium. In this case, it proves particularly beneficial if these devices are configured as throttle devices. Throttle devices may be, for example, fittings, in particular throttle fittings or throttle valves in the evaporator tubes, these fittings reducing the tube inside diameter at a point within the respective evaporator tube.
At the same time, devices for reducing the throughflow in a line system, which includes a plurality of parallel lines and through which flow medium can be supplied to the evaporator tubes of the combustion chamber, also prove to be advantageous. In this case, the line system may also precede an inlet header system of evaporator tubes capable of being acted upon in parallel by flow medium. In this case, for example, throttle fittings or throttle accouterments may be provided in one line or in a plurality of lines of the line system. Such devices for reducing the throughflow of the flow medium through the evaporator tubes makes it possible to bring about an adaption of the throughput of the flow medium through individual evaporator tubes to respective heating of these in the combustion chamber. As a result, temperature differences of the flow medium at the outlet of the evaporator tubes are additionally kept particularly low in a particularly reliable way.
The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically disposed steam generator tubes which are welded to one another in a gastight manner and which are capable of being acted upon in each case in parallel by flow medium.
Adjacent evaporator or steam generator tubes are advantageously welded to one another in a gastight manner via metal bands, so-called fins. The fin width influences the introduction of heat into the steam generator tubes. The fin width is therefore adapted, preferably as a function of the position of the respective evaporator or steam generator tubes in the steam generator, to a heating profile predeterminable on the gas side. In this case, a typical heating profile determined from experimental values or else a rough estimation, such as, for example, a stepped heating profile, may be predetermined as the heating profile. Through the use of the suitably selected fin widths, even when the various evaporator or steam generator tubes are heated to a greatly differing extent, an introduction of heat into all the evaporator or steam generator tubes can be achieved which is such that temperature differences at the outlet of the evaporator or steam generator tubes are kept particularly low. Premature material fatigues are reliably prevented in this way. The steam generator consequently has a particularly long useful life.
The horizontal gas flue advantageously has provided in it a number of superheater heating surfaces which are provided approximately perpendicularly to the main direction of flow of the fuel gas and the tubes of which are connected in parallel for a throughflow of the flow medium. These superheater heating surfaces, provided in a suspended form of construction and also designated as bulkhead heating surfaces, are heated predominantly by convection and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly favorable utilization of the fuel-gas heat is thereby ensured.
The vertical gas flue advantageously has a number of convection heating surfaces which are formed from tubes disposed approximately perpendicularly to the main direction of flow of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly by convection.
In order, furthermore, to ensure a particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer.
The burners are advantageously provided on the end wall of the combustion chamber, that is to say on that containment wall of the combustion chamber which is located opposite the outflow orifice to the horizontal gas flue. A steam generator configured in this way can be adapted particularly simply to the burnup length of the fuel. The burnup length of the fuel is to be understood, here, as the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the fuel. The maximum burnup length for the respective steam generator is obtained, in this case, at the steam power output of the steam generator under full load, the so-called full-load operating mode of the steam generator. The burnup time tA, in turn, is the time which, for example, a coaldust grain of average size requires to burn up completely at a specific average fuel-gas temperature.
In order to keep material damage and undesirable pollution of the horizontal gas flue, for example due to the introduction of high-temperature molten ash, particularly low, the length L of the combustion chamber, defined by the distance from the end wall to the inlet region of the horizontal gas flue, is advantageously at least equal to the burnup length of the fuel in the full-load operating mode of the steam generator. This length L of the combustion chamber will generally be greater than the height of the combustion chamber, measured from the funnel top edge to the combustion chamber ceiling.
In an advantageous embodiment, for the particularly favorable utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is selected as a function of the BMCR value W (given in kg/s) of the steam generator, of the burnup time tA (given in s) of the fuel and of the outlet temperature TBRK (given in xc2x0 C.) of the fuel gas from the combustion chamber. BMCR stands for Boiler Maximum Continuous Rating and is the term conventionally used internationally for the highest continuous power output of the steam generator. This also corresponds to the design power output, that is to say to the power output in the full-load operating mode of the steam generator. In this case, with a given BMCR value W of the steam generator, the higher value of the two functions (1) and (2) substantially applies to the length L of the combustion chamber:
L(W, tA)=(C1+C2xc2x7W)xc2x7tAxe2x80x83xe2x80x83(1)
and
L(W, TBRK)=(C3xc2x7TBRK+C4)W+C5(TBRK)2+C6xc2x7TBRK+C7xe2x80x83xe2x80x83(2)
with
C1=8 m/s and
C2=0.0057 m/kg and
C3=xe2x88x921.905xc2x710xe2x88x924 (mxc2x7s)/(kgxc2x0 C.) and
C4=0.286 (sxc2x7m)/kg and
C5=3xc2x710xe2x88x924 m/(xc2x0 C.)2 and
C6=xe2x88x920.842 m/xc2x0 C. and
C7=603.41 m.
xe2x80x9cSubstantiallyxe2x80x9d is to be understood, here, as a permissible deviation of +20%/xe2x88x9210% from the value defined by the respective function.
The advantages achieved through the use of the invention are, in particular, that, due to the series connection of the first group of evaporator tubes with the second group of evaporator tubes, particularly low temperature differences between adjacent evaporator tubes when the steam generator is in operation are ensured even in a horizontal combustion chamber. In this case, the length of the combustion chamber is configured in such a way that a particularly favorable utilization of the combustion heat of the fossil fuel is ensured. Moreover, configuring the combustion chamber for the fuel gas to flow through in an approximately horizontal direction affords a particularly compact form of construction of the steam generator. This makes it possible, when the steam generator is incorporated into a steam turbine plant, to have particularly short connecting pipes from the steam generator to the steam turbine.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fossil-fired steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.